Selenium rectifier



United States Patent SELE'NIUM RECTIFIER No Drawing. Application DecemherS 1952, I

Serial No. 324,847

f 16 "Claims. (Cl. 317 -241) This invention relates to selenium rectifiers and to the method of making the same,'and particularly to theblocking layer andrmethodof forming the same.

As is well known, selenium rectifiers consist of a layer of selenium in crystalline form disposed betweena carrier electrode and a counterelectrode. Between the selenium and counterelectrodeis a thin blockingor dielectric layer which functions to promote the unilateral conductivity .of the selenium. In order to reduce the resistance of the 'rectifier in the forward or conducting direction, "it is essen- .tial that the blocking layer be as thin .as possible. "In practice, blocking layers have thicknesses of the .ordertof 10 cm. Because of the extreme thinness of such blocking layers, .the variety of materialssuitable ,for use as blocking layers is limited.

Blocking layers may be formed of such materials as lacquer, .zres'inandother dielectrics. As a rule, an insulating .or :dielectric material cannot be used as 'a blocking ilayer successfully simply .because of its insulation characteristic.

lln:ac,cordance with the .present invention, Ihave found nhat ta selenium rectifier .having .a blocking layer formed of a thin film of an organic borate compound has highly desirable properties. The preferred organic borate compounds are low molecular weight normal alkyl borate compound having from 2 to 8 carbon atoms in the molecule. Of these, n-butyl borate and n-amyl borate have given the best results. Higher molecular weight normal alkyl borates, as Well as other organic borate compounds such as iso-alkyl, aryl, alkaryl and aralkyl borates, may also be used. The films of these organic borates exhibit high effective blocking characteristics even when of extreme thinness, as below about 10 cm., providing there is uniformity in film thickness.

Uniformity of blocking layer thickness is of substantial importance. Thus, an area having an abnormally thick blocking layer will effectively have a higher forward resistance at that point. The result will be that the efficiency of the rectifier will suffer and there Will be a tendency for current conduction to occur along the selenium layer rather than through it. This, of course, will further aggravate the operating characteristics of the rectifier and may ultimately suffice to result in hot or burned spots in the rectifier.

In accordance with the present invention, the selenium layer is first coated evenly with a thin film of a substantially non-aqueous solution of an organic borate. The coating is preferably allowed to dry at room temperature, or at an elevated temperature in the order of about 100 to 200 F., thereby depositing a precipitate in the form of a very thin layer of the organic borate uniformly distributed over the surface of the selenium.

The organic borate solution is applied to the selenium when the selenium is in its conducting crystalline form and is ready for the application of a counterelectrode.

The borate compound solution is applied in any desired etc., to obtain the desired the layer of the organic manner, as by spraying, dipping, thin coating of liquid. After followed. Thus,

"200 C. up to about 220 See 2 borate compound has been formed upon the conducting selenium and the layer dried, the counterelectrodematerial may then-be applied in any suitable fashion.

The conventional procedure for the manufacture of a selenium cell, apart from the blocking layer, may *be as is well known, the carrier "electrode may be of iron, nickel-plated iron, aluminum, many other metal suitable forthe purpose. customarily, the carrier electrode has one surface suitably scored or roughened so that a selenium layer will have satisfactory adherence. The 'layer of selenium on the carrier electrode may be obtained'in any one of a num'bero'f ways. For example, it is customary to dispose a layer of dry amorphous selenium upon the carrier electrode and then toheat the selenium to a temperature of between C. and C. while pressure is applied to produce a thin uniform layer of selenium. It is also customary to flow molten selenium upon'the carrier electrode or condense selenium vapors upon the carrier electrode. The selenium layer initially is in the amorphous, non-conducting form. Thereafter, the carrier electrode and layer of amorphous selenium are heat-treated at temperatures 'ofyfromabout C. to convert theselenium into the gray,'crystalline, conducting form.

After the heat conversion treatment, the selenium is provided-with ablocking layer and a counterelectrode is 'then applied. The art has customarily called for spraying suitable metal over the blocking layer to provic'le a counterelectrode. Thus, as an example, Woods metal is widely used for spraying to form a 'counterelec'trode. Other metals, such as cadmium, may also be used. It is, of course, well known that the'choice of metals for counterelectrodes is limited by the fact that the .counterelectrode metal must cooperate withthe selenium for proper action.

Finally, the assembled seleniumcell iselectro'formed is impart thereto the desired unilateral conductivity.

As more fully disclosed and claimed in my copending application, Serial No. 291,771, filed on June 4, 1952, it is also possible to use a solid sheet of counterelectrode metal and apply the blocking layer to the counterelectrode rather than to the selenium. Thereafter, the counterelectrode with the blocking layer is applied over the selenium layer. It is also possible to apply the counterelectrode and blocking layer over the selenium while the selenium is in its amorphous state, and continue the heat treatment of the selenium with the selenium layer permanently disposed between its electrodes. This is disclosed and claimed in the aforementioned copending application.

The organic borate compounds which may be used as blocking layers in accordance with the present invention have the general formula (RO)3B wherein R may be an alkyl, aryl, alkaryl or aralkyl radical. Examples of such organic borates are ethyl borate, n-butyl borate, isobutyl borate, n-amyl borate, isoamyl borate, n-octyl borate, ndecyl borate, phenyl borate and tri-beta naphthyl borate.

The solvent used to make a solution of the organic borate compound may be a lower monohydric alcohol such as ethyl, propyl or butyl alcohol, or any other substantially non-aqueous solvent for the organic borate which will not deleteriously affect the selenium. The solution may be applied in any desired manner, such as by spraying, or dipping, or by centrifuging, for example. Centrifuging involves the spinning of the material upon which the solution is applied so that excess liquid Will be thrown off. Thus, a disc with carrier electrode and selenium may be rapidly spun while a small quantity of solution is applied at the center. Centrifuging may be used in conjunction with spraying, or after dipping, in order to reduce to a minimum the amount of solution remaining upon the surface of the selenium or carrier electrode.

Specific examples of organic borate solutions providing highly effective blocking layers are 2% to 5% Soluti ns of n-butyl borate and n-amyl borate in n-butyl alcohol.

The amounts of the borate compounds in the solutions may vary widely, say from about 0.5% to 40%.

I am not prepared to state what takes place when a thin film of the organic borate is dried upon the selenium, but it appears that the borate undergoes some hydrolysis and/ or polymerization. In any event, the dried film operates effectively to promote the unilateral conductivity of the selenium layer in the electroformed cell. Selenium cells produced with n-butyl borate and n-amyl borate blocking layers are capable of blocking 25 to 30 volts R. M. S.

I claim:

1. In the manufacture of a selenium rectifier assembly having a layer of selenium between the carrier electrode and counterelectrode and a blocking layer between the selenium and counterelectrode components, the method of forming the blocking layer which comprises applying a substantially non-aqueous solution of an organic borate compound having the general formula (RO)3B wherein R is a radical selected from the class consisting of alkyl,

. aryl, alkaryl and aralkyl radicals as a thin film over a surface of one of said components, and then drying the film.

2. In the manufacture of a selenium rectifier assembly having a layer of selenium between the carrier electrode and counterelectrode with a blocking layer between the selenium and counterelectrode components, the method of forming the blocking layer which comprises applying a substantially non-aqueous solution of an alkyl borate compound as a thin film over a surface of one of said components, and then drying the film.

3. The method of claim 2 wherein the alkyl borate compound is a normal alkyl borate compound having from 2 to 8 carbon atoms in the molecule.

4. The method of claim 2 wherein the alkyl borate compound is n-butyl borate.

5. The method of claim 2 wherein the alkyl borate compound is n-amyl borate.

6. The method of claim 2 wherein the alkyl borate compound is ethyl borate. v p

7. The method of claim 1 wherein the borate coinpound is phenyl borate.

8. The method of claim 1 wherein the borate compound is tri-beta naphthyl borate.

9. A selenium rectifier comprising a carrier electrode and a counterelectrode on opposite sides of a selenium layer and a blocking layer of a thin film of an organic borate compound having the general formula (RO)3B, wherein R is a radical selected from the class consisting of alkyl, aryl, alkaryl and aralkyl radicals, between the selenium layer and the counterelectrode.

10. A selenium rectifier comprising a carrier electrode and a counterelectrode on opposite sides of a selenium layer and a blocking layer of a thin film ofan alkyl borate compound between the selenium layer and the counterelectrode.

11. The rectifier of claim 9 wherein the borate is a normal alkyl borate having from 2 to 8 carbon atoms in the molecule.

12. The rectifier of claim 9 wherein the borate is nbutyl borate.

13. The rectifier of claim 9 wherein the borate is namyl borate.

14. The rectifier of claim 9 wherein the borate is ethyl borate.

15. The rectifier of claim 9 wherein the borate is phenyl borate.

16. The rectifier of claim 9 wherein the borate is tribeta naphthyl borate.

References Cited in the file of this patent UNITED STATES PATENTS Re. 19,604 Edenburg June 11, 1935 2,053,474 Graves et al. Sept. 8, 1936 2,137,428 Van Geel et al Nov. 22, 1938 2,221,596 Lorenz Nov. 12, 1940 2,446,254 Van Amstel Aug. 3, 1948 2,475,310 Clark July 5, 1949 

9. A SELENIUM RECTIFIER COMPRISING A CARRIER ELECTRODE AND A COUNTERELECTRODE ON OPPOSITE SIDES OF A SELENIUM LAYER AND A BLOCKING LAYER OF A THIN FILM OF AN ORGANIC BORATE COMPOUND HAVING THE GENERAL FORMULA (RO)3B, WHEREIN R IS A RADICAL SELECTED FROM THE CLASS CONSISTING OF ALKYL, ARYL, ALKARYL AND ARALKYL RADICALS, BETWEEN THE SELENIUM LAYER AND THE COUNTERELECTODE. 